Yeah, well..."entire solutions" are not the best way to learn. You can either check your notes/book(s) or even one of the several millions of sites where this stuff is treated...
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DonAntonioApr 20 '13 at 16:59

Without some additional information, you quite simply can't. The closest you can come is using the identity $$\cos^2\theta=\frac1{\tan^2\theta+1}$$ (which is derived from the Pythagorean identity, and holds wherever $\tan\theta$ is defined), from which you can determine that $$\cos\theta=\pm\sqrt{\frac1{1+\tan^2\theta}}.$$ Now, if you have an additional assumption, such as that $\theta$ is acute, then this becomes $$\cos\theta=\sqrt{\frac1{1+\tan^2\theta}},$$ at which point you can use this together with the angle sum identity for cosine to solve your problem